Variable optical attenuators are widely used in optical telecommunications and other applications to regulate the optical power level of optical signals for equalization purposes or to manage the dynamic range and sensitivity of photodetectors in optical receivers. Means of realizing optical attenuation include the utilization of linear neutral density filters, attenuating prisms, beam blockers, tilting mirrors, and mechanisms for bending or off-setting optical fibers.
In a prior art solution, Smiley et al. (U.S. Pat. No. 6,167,185) disclose an adjustable optical attenuator that preserves the composition of polarization of a beam of light. The beam attenuator has a cross-section along a plane perpendicular to the direction of propagation of the collimated beam of light in the shape of a wedge. The attenuation is varied using a controller for moving the beam attenuator in order to vary a size of the portion of the wedge within the collimated beam of light.
FIG. 1 shows the adjustable optical attenuator 100 consisting of an opaque cone 101 partially blocking a collimated beam of light 102 shown in cross-section. Adjustment of attenuation is effected by linearly moving the opaque cone 101 in a direction of motion 103 in and out of the collimated beam of light 102. A disadvantage of this arrangement is that the direction of motion 103 must be maintained in precise alignment with the center of the collimated beam of light 102, such that the attenuation of one polarization 110 is matched by attenuation of the other polarization 120. The E and H denote the electric and magnetic field orientations of the respective polarizations.
In another example of prior art disclosed by Payne et al. (U.S. Pat. No. 6,801,354) is shown in FIG. 2. An adjustable optical attenuator 200 comprises a symmetrical array of circular apertures 203 in a membrane 202 forming a 2-D diffraction grating, which is employed to eliminate polarization dependent losses when attenuating a beam of incident light 201. FIG. 3 is a top view of an adjustable optical attenuator 300. A symmetrical array of circular apertures 302 forms a 2-D diffraction grating in a reflective silicon nitride membrane 301. Reflective posts 303 are disposed inside each circular aperture 302. By electrostatically raising or lowering the reflective silicon nitride membrane 301 with respect to the reflective posts 303 an attenuation of the beam of incident light reflected from the adjustable optical attenuator 300 is achieved. The circular shape of the apertures 302 is aimed at reducing PDL of the attenuation of the output reflected beam.
The fabrication of the adjustable optical attenuator 300 entails many deposition, photolithographic and etching steps, increasing the cost of manufacture. It also operates in reflection, complicating access to the attenuator output beam. There is no inherent latching property, as the removal of the electric drive voltage releases the electrostatic forces that displace the membrane, resulting in a return of the attenuation to some quiescent value.
In general, a linear response of attenuation to a control signal is normally preferred. A desired controllable attenuation range is up to 30 dB. Polarization dependent loss (PDL) is preferably below 0.2 dB within the 0 dB to 20 dB attenuation range. Wavelength dependent loss (WDL) is preferably 0.3 dB or less for the fiberoptic telecommunications C-band. For some applications, a variable attenuator is preferred, which retains its attenuation setting even if its power supply is turned off, i.e. having a latching property.
An object of the present invention to provide a variable optical attenuator with a widely controllable attenuation range of 30 dB while at the same time maintaining low PDL and low WDL.
It is a further object of this invention to provide a variable optical attenuator with a latching property.
Finally, it is another object of this invention to provide a variable optical attenuator with linear attenuation response to the driving signal.